In recent years, tunnel field effect transistors with low power consumption (TFET, hereinafter referred to as “tunnel transistors”) have been proposed. In the tunnel transistor, a slope of a drain current to a gate voltage in a sub-threshold region (sub-threshold slope) can exceed a theoretical limit value of a slope in a MOSFET.
However, the tunnel transistor has such a problem that there is a large difference between the work function of a source diffusion layer and the work function of a gate electrode, so that a redundant electric field occurs in a gate insulation film in the upper part of the source, and a leakage current is easily generated between the source and the gate.